The present invention relates to a nickel-hydrogen secondary battery. More particularly, it relates to structures of thin film type electrodes and of cells which use the electrode.
In recent years, with the spread of portable electronic equipment, development of alkaline batteries with higher capacity has been eagerly desired. In particular, nickel-hydrogen batteries, which are secondary batteries comprising a positive electrode consisting essentially of an active material containing nickel hydroxide as the main constituent and a negative electrode containing a hydrogen-absorbing alloy powder as the main constituent, are rapidly extending their use as secondary batteries with a high capacity and high reliability.
The positive electrode of alkaline batteries of prior art is described below.
The positive electrode of alkaline batteries can be broadly divided into the sintered type and the non-sintered type. The former electrode is produced, for example, by impregnating a porous sintered nickel substrate of a porosity of about 80% obtained by sintering nickel powder with a nickel salt solution, such as an aqueous nickel nitrate solution, and then immersing the impregnated substrate in an aqueous alkaline solution, thereby to form a nickel hydroxide active material in the porous sintered nickel substrate. This electrode has its limits in attaining high capacity because, since it is difficult to increase the porosity of the substrate further, the amount of the active material loaded cannot be increased further.
The latter non-sintered type positive electrode comprises, for example, as disclosed in JP-A-50-36935, a three-dimensionally continuous spongy porous substrate of a porosity of 95% or more formed essentially of nickel metal and nickel hydroxide of the active material filled in the substrate. This electrode is in wide use at present as the positive electrode of high capacity secondary batteries. In order to attain a higher capacity in the non-sintered type positive electrode, it has been proposed to fill spherical nickel hydroxide in the spongy porous substrate. In this proposal, spherical nickel hydroxide with a particle diameter of several gm to several ten xcexcm is filled into the pores of the spongy porous substrate, which have a pore size of about 200-500 xcexcm. In such a structure, the charge-discharge reaction proceeds smoothly in the nickel hydroxide which is in the vicinity of the nickel metal skeleton and in which the conductive network is maintained, but the reaction does not proceed sufficiently in the nickel hydroxide detached from the skeleton. In this non-sintered type positive electrode, therefore, in order to improve the utilization factor of nickel hydroxide filled into the substrate pores, a conductive agent is used besides nickel hydroxide of the active material thereby to connect the spherical nickel hydroxide particles electrically with one another. The conductive agents used are cobalt compounds, such as cobalt hydroxide and cobalt monoxide, metallic cobalt, metallic nickel, or the like. In this way, in the non-sintered type positive electrode, the active material can be filled in a high density, and hence a higher capacity can be attained than in sintered-type positive electrode.
Next, the negative electrode of nickel-hydrogen secondary batteries using a hydrogen absorbing alloy is described below. In general, alloys of AB5 type or AB2 are used as the negative electrode active material. AB5 alloys are alloys of LaNi5 type in which, usually, the moiety of La has been replaced by Mm (Misch metal) and the moiety of Ni has been replaced by such metals as Co, Mn, Al, Fe and Cr. Misch metals are alloys of lantanide type based on La.
Such negative electrodes are produced by a method which comprises pulverizing an alloy to a predetermined particle diameter, followed by classification, mixing the alloy powder with a conductive agent and a binder into a form of paste, and then coating the paste on a nickel-plated perforated metal plate, followed by drying and pressing to obtain an electrode plate.
In the non-sintered type positive electrode of the above-mentioned structure, however, the addition of a conductive agent is necessary and, moreover, dense filling of spherical nickel hydroxide into the three-dimensional cage-like space of the spongy porous substrate is difficult to attain, and resultantly the capacity density of the positive electrode plate is about 650 mAh/cc. Furthermore, since the substrate itself has a nickel skeleton with a thickness of about 30 xcexcm, when the electrode is used, for example, to construct a cylindrical nickel-hydrogen secondary battery, sometimes broken parts are developed in the skeleton by the bending force applied in winding the electrode plate spirally, and the tip of the broken skeleton pierces through the separator to cause short-circuit with the negative electrode.
In view of the above-mentioned problems, the main object of the present invention is to provide a nickel-hydrogen secondary battery with a high energy density by improving the electrode plate.
To solve the above-mentioned problems, the nickel-hydrogen secondary battery according to the present invention has a structure which comprises a positive electrode comprising nickel foil and an active material layer consisting essentially of nickel hydroxide solid solution powder formed on the foil surface, a negative electrode comprising nickel foil and a hydrogen absorbing alloy powder layer formed on the foil surface, and a porous separator formed essentially of a polymer resin.
By virtue of the above-mentioned structure, nickel hydroxide of the positive electrode active material can be loaded in a larger amount on the nickel foil of the substrate and, moreover, since it is in the form of a thin film, the conductivity of the active material can be maintained merely by adding thereto a minimum amount of a conductive agent, and resultantly the amount of the positive electrode active material can be relatively increased to raise the energy density. Moreover, since the electrode is a thin film electrode in which foil is used as the substrate, neither the breaking of the skeleton due to bending nor the piercing through of the separator by the broken part occurs. Furthermore, since the negative electrode plate is also a thin film electrode using foil as the substrate, the amount of the conductive agent to be added to the negative electrode active material also can be reduced. As a result the amount of the active materials of the nickel-hydrogen secondary battery as a whole can be increased, making it possible to attain a high capacity.
A first aspect of the present invention relates to a nickel-hydrogen secondary battery which has a structure comprising a positive electrode comprising nickel foil and an active material layer consisting essentially of nickel hydroxide solid solution powder formed on the foil surface, a negative electrode comprising nickel foil and a hydrogen absorbing alloy powder layer formed on the foil surface, and a porous separator formed essentially of a polymer resin. The thickness of the nickel foil is preferably 5-20 xcexcm.
A second aspect of the present invention specifies the thickness of the active material layer formed essentially of nickel hydroxide solid solution powder. From the viewpoints of the current-collecting ability and the capacity density of the positive electrode, the thickness is preferably 10-60 xcexcm on one side of the nickel foil surface.
A third aspect of the present invention specifies the thickness of the hydrogen absorbing alloy powder layer formed on the both sides of the nickel foil, the thickness being 19-50 xcexcm on one side of the nickel foil surface.
A fourth aspect of the present invention specifies the separator, which is a porous film or a nonwoven fabric of a polyolefin resin such as polyethylene and/or polypropylene, and has a porosity of 40% or more. Though the separator may be either of film or of nonwoven fabric, a film-formed separator is preferable to obtain a more enhanced thin film structure. Film is less deformed by compressive stress than nonwoven fabric, so that even when the positive electrode and negative electrode become swollen, the amount of liquid retained by a separator made of film does not decrease materially. Moreover, a film-formed separator has a high gas permeability and hence can suppress the increase of internal pressure the battery.
A fifth aspect of the present invention relates to a nickel-hydrogen battery which comprises a positive electrode comprising nickel foil and a powdery substance consisting mainly of nickel hydroxide held on the foil surface, a negative electrode consisting mainly of hydrogen absorbing alloy powder, an alkaline electrolyte and a separator, said powdery substance of the positive electrode comprising as the main constituent a nickel hydroxide powder with an average particle diameter of 1 xcexcm or less containing as solid solution at least one element selected from the group consisting of Co, Zn, Mn, Ca, Cr, Al and Fe, and 1-10% by weight of a cobalt compound powder with an average particle diameter of 1 xcexcm or less admixed with the nickel hydroxide powder. The nickel hydroxide powder containing the above-mentioned metal element as solid solution has an average particle diameter of 1 xcexcm or less, with which a cobalt compound powder with an average particle diameter of 1 xcexcm or less is admixed as a conductive agent in an amount of 1-10% by weight relative to the nickel hydroxide powder. Consequently, difference in particle diameters between the two kinds of powder is very small, and the cobalt compound can disperse uniformly among nickel hydroxide powder particles to form a good conductive network. Moreover, the nickel hydroxide powder can be made to discharge from the surface to the innermost part thereof. Resultantly the capacity density of the positive electrode can be increased.
A sixth aspect of the present invention specifies the cobalt compound used as the conductive agent. The compound preferably consists essentially of cobalt hydroxide or cobalt oxide alone or both of them.
A seventh aspect of the present invention relates to a battery which comprises a positive electrode comprising nickel foil and a powdery substance containing nickel hydroxide as the main constituent held on the foil surface, a negative electrode containing hydrogen absorbing alloy powder as the main constituent, an alkaline electrolyte and a separator, said powdery substance of the positive electrode comprising as the main constituent a nickel hydroxide powder with an average particle diameter of 1 xcexcm or less containing as solid solution at least one element selected from the group consisting of Co, Zn, Mn, Ca, Cr, Al and Fe, and 1-10% by weight of a cobalt compound powder with an average particle diameter of 1 xcexcm or less admixed with the nickel hydroxide powder, the nickel hydroxide powder having a crystallite size of 100 nm or less in the direction parallel to the direction of a form  less than 100 greater than  and 500 nm or less in the direction parallel to the direction of a form  less than 001 greater than  plane. The reason or specifying the size of crystallites which form the above-mentioned nickel hydroxide powder containing a metal as solid solution is that when the crystallite size is larger than the dimension specified above, efficiency in charge-discharge tends to be low and the improvement of utilization factor tends to be difficult to recognize.
An eighth aspect of the present invention relates to a battery which comprises a positive electrode comprising nickel foil and a powdery substance containing nickel hydroxide as the main constituent held on the foil surface, a negative electrode containing hydrogen absorbing alloy powder as the main constituent, an alkaline electrolyte and a separator, the powdery substance of the positive electrode containing as solid solution at least one element selected from the group consisting of Co, Zn, Mn, Ca, Cr, Al and Fe, the amount of the element contained as solid solution being 1-10% by mol. The amount of the element is specified because, since it is generally considered that an element in solid solution basically does not participate in charge-discharge reaction, increase in the amount of element in solid solution, which relatively reduces the nickel content, results in a lower energy density. Thus the amount has its upper limit.
A ninth aspect of the present invention relates to a process for producing a positive electrode, which comprises immersing nickel foil in an aqueous solution containing nickel nitrate and at least one nitrate selected from the group of nitrates consisting of manganese nitrate, iron nitrate, aluminum nitrate, chromium nitrate, calcium nitrate, lanthanum nitrate, copper nitrate, titanium nitrate, magnesium nitrate, yttrium nitrate, silver nitrate, cobalt nitrate and zinc nitrate and forming an active material in the form of thin film on the nickel foil surface by means of electrolytic deposition. The thin film formed by electrolytic deposition tends little to develop pores in the film and hence makes it possible to form a high density active material layer.
A tenth aspect of the present invention relates to a treatment applied after an active material layer of thin film has been formed on nickel foil, the treatment comprising the step of impregnating the active material layer with an aqueous solution of at least one salt selected from the group consisting of the nitrates, sulfates and chlorides of nickel, manganese, iron, aluminum, chromium, calcium, lanthanum, copper, titanium, magnesium, yttrium, silver, cobalt and zinc and the subsequent step of treating the impregnated active material layer with an alkali hydroxide. Since the thin film thus formed is not a film constituted of a single crystal, the above-mentioned metals introduced into the space between crystals serve as a conductive agent or a substance which raises oxygen-generating voltage and thus can improve the utilization factor and characteristic properties of the thin film active material.
An eleventh aspect of the present invention relates to a treatment applied after an active material of thin film has been formed on nickel foil, the treatment comprising, subsequently to the formation of the active material layer, immersing the active material layer in an aqueous solution of at least one nitrate selected from the group of nitrates consisting of nickel nitrate, manganese nitrate, iron nitrate, aluminum nitrate, chromium nitrate, calcium nitrate, lanthanum nitrate, copper nitrate, titanium nitrate, magnesium nitrate, yttrium nitrate, silver nitrate, cobalt nitrate and zinc nitrate to perform electrolytic deposition, whereby the metals of the above-mentioned salts are made to exist on the surface of the thin film active material layer as a conductive agent or a substance which raises oxygen-generating voltage, and resultantly the utilization factor and characteristic properties of the thin film active material layer are improved.
A twelfth aspect of the present invention relates to a process for producing a thin film electrode which comprises the step of forming a conductive film on the surface of active material powder consisting essentially of a spherical nickel hydroxide solid solution with an average particle diameter of 15 xcexcm or less, the step of mixing the active material and a hydrocarbon polymer into the form of paste, the step of coating the paste on nickel foil and the step of rolling the coated nickel foil. In this process, since a conductive material has been applied onto the surface of the active material, the addition of a conductive agent other than the active material is unnecessary and an active material with a high density can be obtained.
A thirteenth aspect of the present invention relates to a process for producing a negative electrode, which comprises the step of mixing spherical hydrogen absorbing alloy powder with an average particle diameter of 15 xcexcm or less coated with nickel, a hydrocarbon polymer and a carbonaceous material or metallic nickel powder to prepare a paste, the step of coating the paste on nickel foil, and the step of rolling the coated nickel foil. Heretofore, hydrogen absorbing alloy powder, as its particle diameter decreases, is apt to be corroded and resultantly to lower its hydrogen-occlusion capacity. According to the present process, however, a hydrogen absorbing alloy powder surface-coated with nickel, which has a high alkali resistance, is used and hence the hydrogen absorbing capacity is not lowered.
An fourteenth aspect of the present invention also relates to a process for producing a negative electrode, in which hydrogen absorbing alloy powder with an average particle diameter of 15 xcexcm or less coated with nickel is used. The hydrogen absorbing occlusion alloy powder used herein is not limited to spherical hydrogen absorbing alloy described in claim 11 but may also be non-spherical powder obtained, for example, by grinding cast hydrogen absorbing alloy, followed by sizing.
A fifteenth aspect of the present invention described relates to a typical process for producing a negative electrode, which comprises the step of forming a thin film layer of a hydrogen absorbing alloy described below on nickel foil by means of sputtering using as the target an AB5 type hydrogen absorbing alloy comprising a Misch metal consisting essentially of La, Ce, Pr, Nd and Sm as the A-site and Ni, Co, Mn and Al as the B-site, and the step of annealing the nickel foil having the hydrogen absorbing alloy layer formed thereon. Since the hydrogen absorbing alloy layer is formed by means of sputtering, the addition of conductive agents and additives for bonding hydrogen absorbing alloy which has been required in previous coating-type negative electrode becomes utterly unnecessary and a high density loading of the active material becomes possible.
The sixteenth aspect of the present invention relates to a process which comprises the step of forming a thin film layer of a hydrogen absorbing alloy described below on nickel foil ky means of sputtering using as the target AB5 type hydrogen absorbing alloy comprising La, Ni, Co, Mn and Al, and the step of annealing the nickel foil having the hydrogen alloy layer formed thereon. Thus, the process is a process wherein the Misch metal which is one of the constituents of the hydrogen absorbing alloy described is replaced by La. It gives a negative electrode with characteristic properties equal to those obtained by the process described.
A seventeenth aspect of the present invention relates to a process which comprises the step of forming a thin film layer of hydrogen absorbing alloy on nickel foil by means of sputtering using an alloy comprising La, Co, Mn and Al as the target and the step of annealing the nickel foil having the hydrogen absorbing alloy layer formed thereon. By annealing, the nickel foil forms an alloy with la, Co, Mn and Al, thereby to change itself into a negative electrode active material.
An eighteenth aspect of the present invention relates to a process which comprises the step of mixing the nitrate, sulfates or chlorides of La, Ni, Co, Mn and Al in a predetermined mixing ratio to prepare an aqueous solution of the mixture, the step of performing electrolytic plating on nickel foil in the aqueous solution prepared above and the step of annealing the nickel foil having a plating thin film formed thereon. The plating film comprising La, Ni, Co, Mn and Al as the main constituents formed on the nickel foil is turned into a negative electrode active material by annealing.
A nineteenth aspect of the present invention relates to a process which comprises the step of mixing the nitrates, sulfates or chlorides of La, Co, Mn and Al in a predetermined mixing ratio to prepare an aqueous solution of the mixture, the step of performing electrolytic plating on nickel foil in the aqueous solution prepared above and the step of annealing the nickel foil having a plating thin film formed thereon. By annealing, the nickel foil forms an alloy with La, Co, Mn and Al to turn into a negative electrode active material.
A twentieth aspect of the present invention relates to a process which comprises the step of melting a Misch metal comprising La, Ce, Pr, Nd and Sm, and Ni, Co, Mn and Al in a melting furnace, the step of rapidly quenching the molten body in an inert gas atmosphere by the single roll method or the twin roll method to obtain a thin film, the step of mixing minute strips of the thin film obtained, a hydrocarbon polymer, and a carbonaceous material or metallic nickel powder to form a paste, the step of coating the paste on nickel foil and the step of rolling the coated nickel foil. In this process, thin film in the form of strips about 10 xcexcm in thickness is coated on nickel thin film, so that the process also makes it possible to attain high density loading of the active material.
Embodiment of the Invention in the Form of Battery
A nickel-hydrogen secondary battery useful in practice, which uses the above-mentioned positive electrode comprising nickel foil and an active material layer consisting essentially of nickel hydroxide solid solution powder formed on the nickel foil surface, the above-mentioned negative electrode comprising nickel foil and a hydrogen absorbing alloy layer formed on the nickel foil surface, and, as a separator, porous thin film or nonwoven fabric formed essentially of a polymer resin, can be formed by arranging the separator so as to insulate the positive electrode plate from the negative electrode, winding the whole spirally to form an electrode plate assembly, putting the assembly in a battery case, then pouring a prescribed amount of electrolyte into the case, and sealing the case opening with a sealing plate.